Such a method is, for example, previously known from U.S. Pat. No. 4,464,336. However, it has proved that sterilization using flash light is not wholly effective in packaging materials with a high bacteria coating or contamination, probably because certain surface layers which contain bacteria or microorganism colonies are "shadowed" by superjacent bacteria colonies. As a result of this shadowing effect, not all bacteria or microorganisms located on the packaging material surface are reached during the treatment process. By producing, in the manner disclosed in the appended Claims, a packaging material with a transparent plastic inside and a subsequent light-reflective coating, it is possible to re-reflect substantial proportions of the emitted light which will then impinge upon the packaging material surface from "the opposite direction" as compared with direct light treatment and, thereby, to affect even those bacteria most proximal the surface layer which are not exposed to the direct effects of the light from the flash bulb. Naturally, this secondary light effect will not be as strong as the direct light effect, since the light will, first, be obliged to travel a longer distance, secondly pass through the transparent plastic surface layer twice, and thirdly be reflected by the light-reflective layer. However, by employing a plastic layer which is as transparent as possible, and by utilizing as good a light-reflective capacity as possible in the reflective layer, it is possible at those photoenergy levels which are employed, to obtain a reflected light wave which has a photoenergy of up to 90 percent of the initial energy.